In recent years, various types of auxiliary systems are provided in vehicles to implement smooth driving of the vehicles and reduce the fuel economy thereof. One of such auxiliary systems is a power steering system. The power steering system enables comfortable driving of a vehicle and makes the operation thereof easy and appropriate.
The power steering system assists steering operation with a motor (an electric motor) to improve the fuel economy of vehicles. As the power steering system driven by the motor, vehicles are normally equipped with a device including an electric power steering system (hereinafter, referred to as an EPS system) (see Patent Literature 1, for example). Heavy vehicles, which cannot be equipped with the EPS system, are often equipped with an electric pump power steering system (hereinafter, referred to as an EHPS system).
However, equipment of the EPS or EHPS system causes problems of deterioration of the fuel economy resources due to electric power consumption for separately driving the electric motor and deterioration of the mountability to vehicles.
On the other hand, vehicles are increasingly equipped with an idling stop system (hereinafter, referred to as an ISS) as a technique to improve the fuel economy. However, in vehicles equipped with transmissions that use hydraulic pressure, such as automatic transmissions (AT), automated manual transmissions (AMT), or continuously variable transmissions (CVT), the engine is stopped by the ISS, and hydraulic pressure cannot be supplied to the transmission that uses hydraulic pressure while the engine is in idling stop mode.
Until the supply of hydraulic pressure is started after the engine starts, it takes about one second, and hydraulic pressure cannot be obtained just after the start of the engine. It is therefore necessary to supply hydraulic pressure to the transmission that uses hydraulic pressure to ensure driving force instantaneously after the engine is recovered from the idling stop mode.
Accordingly, there is a system to supply stand-by hydraulic pressure to the transmission that uses hydraulic pressure by using an electric pump during idling stop mode. This system can quickly ensure the driving force at the start from the idling stop mode by using the electric pump to supply stand-by hydraulic pressure to the transmission which is not supplied with hydraulic pressure from a normal hydraulic pressure supply system because of the stop of the engine while the ISS is in operation, that is, during idling stop mode.
However, the aforementioned system to supply the stand-by hydraulic pressure needs to be separately provided with the electric pump and has a problem of deterioration of the mountability to the vehicle. Moreover, there is also a problem that operating the system to supply the stand-by hydraulic pressure during idling stop mode increases the electric power consumption during idling stop mode.
According to a graph showing the influence on electric power consumption during idling in FIG. 4, when a large amount of electric power is consumed during the idling stop mode, the amount of power generated by an alternator increases in order to charge the battery after the engine is restarted. This increases the load on the engine and increases the fuel consumption, thus reducing the real effect by the ISS.
That is, the ISS, which is mounted in order to improve the fuel economy, causes a problem that driving force cannot be obtained instantaneously after the engine is restarted. When the system to supply stand-by hydraulic pressure to the transmission that uses hydraulic pressure is mounted in order to solve the above problem, current of about 3.0 to 4.0 A is required while the vehicle is in idling stop mode, thus causing another problem of reduction of the effect on improving the fuel economy.
Accordingly, when a vehicle is equipped with both the EHPS system and the system to supply stand-by hydraulic pressure, the amount of current is increased by about 4.0 to 6.0 A in total while the vehicle is in the idling stop mode. Here, according to a graph representing the influence of electric powers of the alternator and battery in FIG. 5, when the amounts of current of the alternator and battery are increased by 1 A, the fuel economy deteriorates by about 0.5 to 0.7%. Accordingly, when the both systems are provided on a vehicle, as represented in FIG. 4, the fuel economy reduction effect of 3 to 7% is reduced, and the effect of the ISS can be canceled in the worst case. Accordingly, it is necessary to minimize the amount of current while the vehicle is in the idling stop mode as much as possible in order not to reduce the fuel economy reduction effect obtained by equipment with the ISS.
As described above, in order to equip a vehicle with the auxiliary system, it is necessary to carefully consider the problems of deterioration of the mountability to the vehicle and deterioration of fuel economy due to reduction of fuel resources by the electric power consumption increased.